On the representation of words and nonwords in visual electrophysiology

Psychophysiology, 46 (2009), 191–199. Wiley Periodicals, Inc. Printed in the USA.
Copyright r 2008 Society for Psychophysiological Research
DOI: 10.1111/j.1469-8986.2008.00753.x
On the representation of words and nonwords in visual
short-term memory: Evidence from human
electrophysiology
DAVID PREDOVAN,a,b DAVID PRIME,a,b MARTIN ARGUIN,a,b FRÉDÉRIC GOSSELIN,a,b
ROBERTO DELL’ACQUA,c,d and PIERRE JOLICŒURa,b
a
Département de Psychologie, Université de Montréal, Montréal, Québec, Canada
Centre de Recherche en Neuropsychologie et Cognition, Université de Montréal, Montréal, Québec, Canada
c
Department of Developmental Psychology, University of Padova, Padova, Italy
d
Center for Cognitive Science, University of Padova, Padova, Italy
b
Abstract
Electrophysiological measures were used to investigate the contribution of lexical status on the maintenance of letter
strings in visual short-term memory (VSTM). The sustained posterior contralateral negativity (SPCN), an electrophysiological marker of storage in VSTM, was measured for words and nonwords as well as scrambled letters. A
smaller SPCN was found for words than for nonwords (independently of their pronounceability), indicating that
lexical status inﬂuences storage in VSTM. One possibility is that words produce a smaller SPCN because they can be
recoded to a form that does not require a low-level representation in VSTM. For exploratory purpose, a comparison
between the nonwords and the scrambled nonwords was also made. Based on previous research, the SPCN component
should not be affected by the size of the region enclosing to-be-encoded objects. Surprisingly, signiﬁcant differences
between the SPCN for nonwords and scrambled letters conditions were found, suggesting that special encoding
mechanisms may be recruited to encode word-like letter strings.
Descriptors: Sustained posterior contralateral negativity (SPCN), Visual short-term memory (VSTM), Lexical status,
Electroencephalography, Event-related potentials (ERPs), Reading
stimuli in the other visual hemiﬁeld. Klaver, Talsma, Wijers,
Heinze, and Mulder (1999) argued that the SPCN (or contralateral negative slow wave) reﬂects activity related to encoding
and retention in VSTM, a view that has recently received empirical support from the work of Vogel and colleagues (Vogel &
Machizawa, 2004; McCollough, Machizawa, & Vogel, 2007). In
the work of Vogel and colleagues, a centrally displayed arrow
stimulus cued participants to encode the stimuli appearing in
either the left or right visual hemiﬁeld. A target display consisting
of simple visual stimuli (e.g., colored squares) was then presented
for 100 ms. After a retention interval of 900 ms, another set of
stimuli was presented and participants decided whether or not the
second set was the same as the ﬁrst in the encoded hemiﬁeld.
Starting about 300 ms following the presentation of the target
display, the ERP was more negative at posterior electrodes contralateral to the cued visual ﬁeld (e.g., the voltage was more
negative at electrode PO8 than at PO7 for stimuli encoded from
the left visual ﬁeld). This lateralized voltage difference was sustained during the entire retention interval. Importantly, the amplitude of the voltage difference increased as the number of target
items increased, reaching a maximum when the number of stimuli to be encoded equaled or exceeded the estimated capacity of
VSTM (on a subject-by-subject basis; Vogel & Machizawa,
2004). Furthermore, the voltage difference was smaller on incorrect response trials relative to correct trials, suggesting that
There are reasons to believe that the lexical status of a letter string
would interact with the degree to which visual short-term memory (VSTM) may be engaged during the retention of such strings.
For example, Cowan (1996) postulated that access to a long-term
memory code should facilitate the retention of word stimuli in
working memory. We expected that a similar effect might be
found when examining the role of VSTM (rather than a more
abstract form of memory), but, to our knowledge, no one has
used so far a measure of brain activity speciﬁcally related to
VSTM to examine the inﬂuence of lexical status. In the present
study we utilized event-related potentials (ERPs) to examine this
issue.
Recently, an ERP component, which we refer to as sustained
posterior contralateral negativity (SPCN; Jolicœur, Dell’Acqua,
Sessa, & Robitaille, 2006b), has been suggested to reﬂect information storage in VSTM. The SPCN is observed following the
visual encoding of a stimulus presented off the vertical midline,
either in the left or right visual hemiﬁeld. To deconfound memory
encoding from low-level stimulus differences, the target stimuli in
one visual ﬁeld are presented with an equivalent set of distractor
Address reprint requests to: David Predovan, Département de Psychologie, Université de Montréal, C.P. 6128 Succursale Centre-ville,
Montréal, QC H3C 3J7, Canada. E-mail: [email protected]
al.ca
191
192
this activity contributes to accurate performance (see also
Dell’Acqua, Sessa, Jolicœur, & Robitaille, 2006; Jolicœur,
Dell’Acqua, Sessa, & Robitaille, 2006a; Jolicœur et al., 2006b;
Robitaille, Jolicœur, Dell’Acqua, & Sessa, 2007, for strong associations between behavioral accuracy and the amplitude of the
SPCN in the context of the attentional blink paradigm).
Previous studies that have investigated the relationship between VSTM and the SPCN have used very simple stimuli, such
as colored squares and oriented bars. Here, we study how VSTM
involvement might vary for stimuli such as words, for which we
have preexisting long-term memory (LTM) representations,
compared to groups of random letters (forming nonwords). Our
approach is to measure the SPCN component elicited when subjects encode and remember letter strings that form either a word
or a nonword. We hypothesize that lexical status of the words
should reduce the load in VSTM in comparison to the nonword
condition, because of the availability of a supplementary form of
representation (in LTM), and that this reduction will be reﬂected
in a signiﬁcant attenuation of the SPCN component in the word
condition. Perhaps such stimuli can be encoded directly from a
form of iconic memory to a very abstract level that may not
require the protracted storage of words in VSTM or may require
it to a reduced extent compared to different visual stimulation (e.g., via immediate phonological recoding [Holcomb &
Grainger, 2006, 2007] or recoding to an abstract representation
in semantic memory; see the Discussion for alternative hypotheses concerning this issue). There is good evidence that representations in LTM interact with processing at earlier levels
(letters, features) to produce effects such as the word superiority
effect (Cattell, 1886; McClelland & Rumelhart, 1981) or the letter superiority effect (Reingold & Jolicœur, 1993).
Our expectation that lexical status of stimuli may affect early
visual representations in VSTM is predicated on a subﬁeld of the
neurophysiological literature, mainly based on electroencephalographic (EEG) and magnetoencephalographic (MEG) recordings, showing that lexical access, under particular conditions,
may be extremely rapid. For example, Sereno, Rayner, and
Posner (1998) presented participants with high-frequency or lowfrequency regular and irregular English words. ERPs timelocked to these words during performance in a lexical decision
task differed as a function of condition with effects of word frequency that started as early as 132 ms post-target and continued
into the N1 time range ( 170 ms). These effects suggest that
lexical access took place within this time range on a signiﬁcant
number of trials. Pulvermüller, Assadollahi, and Elbert (2001)
recorded MEG activity while participants performed a lexical
decision task on distinct sets of words that varied in strength of
semantic association. Differences in the event-related magnetic
ﬁelds across the different sets of words were apparent as early as
150 ms following word onset, with the magnetic ﬁeld responses
being particularly pronounced for words characterized by strong
semantic association. These ﬁndings also suggest fast semantic
access, consistent with Sereno et al.’s ﬁndings (see also Hauk,
Davis, Ford, Pulvermüller, & Marslen-Wilson, 2006; Pulvermüller, 2001, for similar conclusions). Note that semantic effects
on event-related magnetic ﬁelds in the Pulvermüller studies were
strongest at sensors over left infero-temporal brain regions (see
also Pulvermüller, Shtyrov, & Ilmoniemi, 2005).
It should be mentioned that this prediction may appear not
entirely congruent with the different view that one reliable indicator of lexical/semantic processing of words would be reﬂected
in the centro-parietal N400 component, that usually begins at
D. Predovan et al.
about 350 ms poststimulus, and it is not, contrary to the SPCN,
characterized by a scalp distribution contralateral to the eliciting
stimuli (e.g., Heil, Rolke, & Pecchinenda, 2004; Rolke, Heil,
Streb, & Hennighausen, 2001; Vogel, Luck, & Shapiro, 1998).
However, as noted by Dell’Acqua, Pesciarelli, Jolicœur, Eimer,
and Peressotti (2007), Holcomb (1993), and Pesciarelli et al.
(2007), it is likely that N400 activity may reﬂect semantic integration of sequential verbal stimuli rather than semantic/lexical
access per se. N400 activity modulations are normally found
when the eliciting stimulus is semantically evaluated following
the prior activation of a context, either via the presentation of a
prime stimulus (e.g., Holcomb & Grainger, 2006) or via the inclusion of the word stimulus in a sentence, as done originally in
seminal work on the N400 (e.g., Kutas & Hillyard, 1980).
Based on evidence suggesting a rapid access of lexical information from words (and also from pictures; see Thorpe, Fize, &
Marlot, 1996) presented in isolation, we used electrophysiological recordings to study potential interactions between lexical
access mechanisms and mechanisms responsible for the encoding
and maintenance of representations in VSTM. In addition to a
comparison of strings of three letters that formed a word or a
nonword, we also presented nonwords in which letters were misaligned horizontally (scrambled letters condition) to determine if
the spatial layout of letter stimuli can have consequences for the
SPCN. McCollough et al. (2007) showed that the SPCN for
colored squares is unaffected by the size of the region in which the
squares were presented, suggesting that the spatial layout of
simple stimuli does not inﬂuence the SPCN. We thought it was
important to verify this supposition for letters, however, because
of our extensive reading experience that may have produced
specialized mechanisms for dealing with compact horizontal letter strings. Operations such as chunking of groups of letters,
leading to representations of pairs or triads of letters (e.g., ‘‘the’’)
could reduce the need for the retention of the visual features of
the stimuli themselves. In short, specialized encoding mechanisms built up over many years of practice may affect the nature
of subsequent representations in VSTM.
Method
Participants
Sixty two native French speakers (ages 18–29 years, mean 5 22.5
and SD 5 3) participated in this experiment. All participants
were naı̈ve volunteers and were paid 20 Canadian dollars. All
participants reported normal or corrected-to-normal vision and
no history of neurological problems. Thirty individuals participated in the pronounceable nonword condition and 32 individuals participated in the nonpronounceable nonword condition.
Seven participants from the pronounceable nonword condition
and 8 participants from the nonpronounceable nonword condition were excluded from data analysis because an excessive number of trials were rejected mainly due to a tendency to shift their
gaze toward the attended hemiﬁeld but also due to EEG artifacts.
On average, subjects who were retained for detailed analyses
moved their eyes less than 0.151 of visual angle in the direction of
the target.
Task and Design
The experimental task required participants to encode and retain
the identity of three visually presented letters. The target letters
193
Lexical status and VSTM
Nonwords Scrambled
A
E
+
+
X
B
Words
A
E
Nonwords
Q
U
+
A
E
+
D
T
+
+
850 ms
X
E
S
L
+
M
SEL
+
C OQ
ESL
+
MDY
D
150 ms
+
800 ms
200 ms
+
Time
Figure 1. Sequence of events in each trial for the word, nonword, and nonword scrambled conditions.
were equally likely to be presented in the left or right visual ﬁeld.
A retention period of 850 ms ensured that the task could not
easily be carried out on the basis of a very low-level form of
memory such as visible persistence (Coltheart, 1980). After the
retention interval the participants performed a two-alternative
forced-choice identiﬁcation test on the identity of one of the letters. A 2 3 between-within design was employed. The withinparticipant manipulation varied the lexical status of the target
letters. In the word condition the three target letters were grouped
into a horizontal string that formed a French word. In the nonword condition the target letters were grouped into a horizontal
string that did not form a valid word. In the scrambled letters
condition the letters were not aligned horizontally and did not
form a word. The between-participants manipulation varied the
pronounceability of the letters in the nonword condition. In the
pronounceable condition, to which about half the participants
were submitted, the nonwords had a pronounceable sequence of
consonants and vowels that closely matched the proportions of
strings with particular sequence structure in the word condition
(i.e., CVC, VCC, CCV, etc.). In the unpronounceable condition
the nonwords were random consonant strings.
Stimuli
The visual ﬁeld cue was a centrally presented 0.21 gray arrow
pointing toward the left or right. Target displays consisted of
three letters presented on each side of the vertical meridian (six
letters total). The letter stimuli were 0.71-tall capital letters (A–Z)
presented in gray on a black background. In the word and nonword conditions the letter strings were centered 2.41 to the left
and right of ﬁxation on the horizontal meridian. The horizontal
center-to-center distance between each letter was 1.31. In the
scrambled condition the letters had the same horizontal arrangement as in the other two conditions but each letter was randomly
displaced, upward or downward, by 0.951 or 1.901. The test displays had the same spatial arrangements as the target displays,
except that one letter in each visual ﬁeld was probed by two test
letters positioned directly above and below its former location
(Figure 1). The horizontal position of the test letters was always
different in the left and right visual ﬁelds. The other letters were
replaced by gray oval outlines. One of the two test letters
matched the identity of the letter at the probed position in the
target display. In the word condition both test letters formed
valid words in conjunction with the other letters in the target
display. For example, in English, if the target letters had been
SUN, the identity of the ﬁrst letter could be tested with the letters
S and F, each of which forms a word. Furthermore, the distractor
letter string in the opposite hemiﬁeld was always of the same type
as the string in the attended hemiﬁeld (i.e., word, nonword, or
scrambled letters).
Experimental Procedure
The trial sequence is illustrated in Figure 1. Participants viewed
the experimental stimuli displayed on a computer monitor from a
distance of 57 cm and a chin rest was used to stabilize the head.
Each trial began with the presentation of a central ﬁxation cross
that remained on the screen throughout the trial. Participants
were instructed to maintain their gaze at ﬁxation during a trial,
and to blink only between trials. An arrow cue indicating the
visual ﬁeld of the to-be-remembered letters replaced the ﬁxation
cross for 200 ms. After a further 800-ms delay, the target display
was presented for 150 ms, followed by a blank screen with the
ﬁxation cross. One second after the onset of the target display,
the test display was presented. Participants were required to indicate which of the two test letters in the to-be-remembered visual
ﬁeld matched the letter in the target display. To select the letter,
participants had to respond by pushing predetermined keys on
the keyboard (standard Qwerty natural keyboard). Half of the
subjects responded with the right hand (key ‘‘j’’ to select top letter
and ‘‘n’’ to select bottom letter) and the other half responded
with the left hand (keys ‘‘g’’ and ‘‘b’’). Accuracy feedback was
provided immediately after the response in the form of a plus sign
194
for correct responses or a minus sign for errors. Participants
initiated the next trial by pressing the space bar.
The experimental session consisted of 18 practice trials and
720 experimental trials (240 trials per condition) divided into two
blocks of 360 trials. Participants were required to rest between
blocks and could rest at any time during the experiment. The
experimental session lasted approximately 1.5 h.
Electrophysiological Recording and Analysis
The electroencephalogram (EEG) was recorded with active Ag/
AgCl electrodes (Biosemi Active Two) mounted on an elastic cap
at standard 10–10 scalps sites (Fp1, Fpz, Fp2, AF7, AF3, AFz,
AF4, AF8, F7, F5, F3, F1, Fz, F2, F4, F6, F8, FT7, FC5, FC3,
FC1, FCz, FC2, FC4, FC6, FT8, T7, C5, C3, C1, Cz, C2, C4,
C6, T8, TP7, CP5, CP3, CP1, CPz, CP2, CP4, CP6, TP8, P9, P7,
P5, P3, P1, Pz, P2, P4, P6, P8, P10, PO7, PO3, POz, PO4, PO8,
O1, OZ, O2, Iz, M1, and M2). Eye position was monitored by
both the horizontal and vertical electro-oculogram (EOG). The
vertical EOG was recorded as the voltage between Fp1 and an
electrode placed below the left eye. The horizontal EOG was
recorded as the voltage between electrodes placed lateral to the
external canthi. Ampliﬁed EEG and EOG channels were lowpass ﬁltered at 67 Hz and digitized at 256 Hz. After acquisition the EEG channels were referenced to the average of the left
and right mastoids and high-pass ﬁltered at 0.01 Hz (half power
cutoff).
Trials containing blinks, eye movements, and EEG artifacts
were removed prior to ERP averaging by applying automated
artifact detection routines. Participants who had less than 60%
of trials remaining in one or more conditions after artifact
rejection were excluded from further analysis. Separate ERP averages were calculated from EEG epochs time-locked to the presentation of the target display for each experimental condition
and visual ﬁeld of the target items. The resulting ERP averages
were low-pass ﬁltered at 6 Hz and baseline corrected relative to
mean voltage of the 200-ms prestimulus interval. To isolate the
SPCN activity, ERP waveforms from electrodes ipsilateral to the
target items were subtracted from those from contralateral electrodes and the resulting difference waves for each visual ﬁeld
were averaged. For purposes of statistical analysis, pooled SPCN
waves were formed in order to improve the signal-to-noise ratio
of the data. The pooled SPCN waves were derived from electrode
pairs PO7/PO8, O1/O2, and P7/P8. These electrode locations
were chosen because they correspond to the location of the maxima of the SPCN topography (Figure 4b, below). The SPCN
amplitude for each condition was quantiﬁed as the mean voltage
of the pooled SPCN wave in the 400–1000-ms latency range.
Results and Discussion
Behavior
Mean percent correct and mean SPCN amplitude measures were
submitted to separate 2 3 between-within analyses of variance
(ANOVAs) with factors of Group (pronounceable nonword,
unpronounceable nonword) and Lexical Status (word, nonword,
scrambled). The Geisser–Greenhouse correction procedure was
used for repeated-measures involving more than one degree of
freedom.
Mean accuracy rates for the word, nonword, and scrambled
conditions were, respectively, 95%, 94%, and 89% for the pronounceable nonword group and 94%, 88%, and 83% for the
unpronounceable nonword group. The ANOVA of accuracy
D. Predovan et al.
rates revealed a signiﬁcant main effect of Lexical Status, F(2,90) 5
78.6, p o .001, e 5 .67, Zp 2 ¼ :64. In addition, both the main
effect of Group (mean accuracy of 93% for the pronounceable
group vs. 88% for the unpronounceable group), F(1,45) 5 6.8,
p o .02, Zp 2 ¼ :13, and the interaction effect, F(2,90) 5 5.9, p o
.02, e 5 .67, Zp 2 ¼ :12, were signiﬁcant.
ERP
Figure 2 shows grand average waveforms, collapsed across both
groups, separately for stimuli encoded from the left and right
visual ﬁelds at all electrode sites. The word condition is shown in
Panel a, the nonword condition in Panel b, and the nonword
scrambled condition in Panel c. The SPCN can be seen as a
relative negative shift in the ERP wave at electrode sites contralateral to the visual ﬁeld of the target. As expected, the SPCN
difference is largest at lateral occipital sites (see also the topographic maps in Figure 4b). Furthermore, the SPCN difference
can be seen in all three conditions for stimuli encoded from both
visual ﬁelds.
Figure 3 shows the pooled response at three posterior electrode sites for stimuli encoded from the left visual ﬁeld or right
visual ﬁeld for left-sided and right-sided electrodes, for each
stimulus type (words, nonwords, and scrambled letters). As is
evident in the ﬁgure, there was no evidence of left–right asymmetries in the SPCN waveforms for any of the stimulus types.
Figure 4a shows the SPCN subtraction waves (contralateral –
ipsilateral) pooled over electrode pairs PO7/PO8, O1/O2, and
P7/P8 for each group and condition. Figure 4b shows the scalp
distributions of the SPCN for each type of letter string, averaging
over the two groups. These scalp distributions are based on the
SPCN subtraction waves at each lateralized electrode pair (zero
voltage assumed at midline electrodes) and they exhibit the
same relatively focused voltage peak at posterior electrodes (e.g.,
Brisson & Jolicœur, 2007). The distributions were quite similar in
general topography, suggesting that the same component was
elicited in the three string type conditions, but with different
amplitudes.
The SPCN amplitudes were subjected to an ANOVA of the
same type as used for the accuracy results. SPCN amplitude was
highest for the scrambled condition, intermediate for the nonword condition, and smallest for the word condition, producing a
signiﬁcant main effect of Lexical Status, F(2,90) 5 33.9, p o
.001, e 5 .80, Zp 2 ¼ :19. In contrast to the accuracy results, neither the main effect of Group, F(1,45) 5 0.84, p4.36, Zp 2 ¼ :02,
nor the interaction effect, F(2,90) 5 1.0, p4.35, e 5 .80,
Zp 2 ¼ :02, approached signiﬁcance. The lack of any Group
effects indicates that the pronounceability of the nonwords did
not affect the amplitude of the SPCN.
To explore further the effect of Lexical Status on the amplitude of the SPCN, 2 2 between-within ANOVAs were performed for each pairing of the Lexical Status variable. Analysis
of the difference between the word and nonword conditions revealed that the words elicited a signiﬁcantly smaller SPCN than
nonwords, F(1,45) 5 9.8, p o .01, Zp 2 ¼ :18. Neither the main
effect of Group nor the interaction effect approached signiﬁcance, both ps4.62, indicating that the pronounceability of the
nonwords had no effect on the difference in SPCN amplitude
between words and nonwords.
Analyses of the difference in SPCN amplitude between the
scrambled and the other two conditions revealed that the scrambled condition elicited a signiﬁcantly larger SPCN than either
the word condition, F(1,45) 5 48.2, p o .001, Zp 2 ¼ :52, or the
Lexical status and VSTM
195
A
Words LVF
Words RVF
B
Nonwords LVF
Nonwords RVF
Figure 2. A: Grand averaged waveforms, collapsed across groups, at all electrode sites for the word condition. Separate waveforms are shown for stimuli
encoded from the left and right visual ﬁelds. B: Grand averaged waveforms, collapsed across groups, at all electrode sites for the nonword condition.
Separate waveforms are shown for stimuli encoded from the left and right visual ﬁelds. C: Grand averaged waveforms, collapsed across groups, at all
electrode sites from the nonword scrambled condition. Separate waveforms are shown for stimuli encoded from the left and right visual ﬁelds.
196
D. Predovan et al.
c
Nonwords Scrambled LVF
Nonwords Scrambled RVF
Figure 2. Continued
nonword condition, F(1,45) 5 28.1, p o .001, Zp 2 ¼ :38. Neither the main effect of Group nor the interaction effect approached signiﬁcance in either ANOVA, all ps4.25.
An inspection of Figure 4a reveals that the SPCN appears to
begin at a shorter latency in the scrambled condition than in the
other to two conditions. To analyze this effect, SPCN onset was
measured using a fractional area latency measure (Kiesel, Miller,
Jolicœur, & Brisson, 2008) on jackknife-subsample average
waveforms (Ulrich & Miller, 2001). The SPCN onset for each
condition was deﬁned as the point in time at which the area under
Figure 3. Pooled response at left-sided and right-sided posterior electrode sites for stimuli encoded from left visual ﬁeld or right visual
ﬁeld, for each stimulus type (words, nonwords, and scrambled letters).
197
Lexical status and VSTM
Non-Pronounceable
Pronounceable
A
0
1000
2.0 µV
0
1000
2.0 µV
Word
Nonword
Scrambled
B
Word
Nonword
Scrambled
Figure 4. A: Contralateral minus ipsilateral waveforms showing the SPCN for the pooled response at three posterior electrode sites
for each type of stimulus (words, nonwords, and scrambled letters) for each group. B: Scalp distributions of the SPCN for each
stimulus type (mean activity from 400–1000 ms after memory stimulus onset).
the SPCN wave reached 20% of the total area in the interval
between the start of the negative portion of the curve and 500 ms
(this interval corresponds to the growing portion of the SPCN
wave). Analysis of the onset latencies revealed a signiﬁcant main
effect of Lexical Status, F(2,90) 5 5.5, p o .01. Neither the main
effect of Group, F(1,45) 5 1.3, p4.26, nor the interaction effect,
F o 1, approached signiﬁcance. Subsequent analyses revealed
that the SPCN onset earlier in the scrambled condition (332 ms)
than either the word (398 ms), F(1,45) 5 7.3, p o .01, or the
nonword (380 ms), F(1,45) 5 13.4, p o .001, conditions. The
difference in SPCN onset between the word and nonword conditions was not signiﬁcant, F o 1.
The present results were clear-cut in demonstrating that the
lexical status of a letter string modulated activity in VSTM during the retention interval of a VSTM task. When the target string
formed a word, the amplitude of the SPCN was reduced relative
to when the target string did not form a word. At the functional
level, one viable explanation for this effect is that the storage of
target letters that form words is facilitated by a lexically driven
grouping or chunking mechanism that reduces the information
load in VSTM (e.g., Martin, Nazir, Thierry, Paulignan, &
Demonet, 2006). In this view, the availability of an alternative
memory code, already stored in LTM, would reduce the need to
rely on an earlier, lower level visual representation of the letters
by, for instance, providing supplementary top-down activation
support to word representations in VSTM. A second possibility
is that horizontal letter strings may make contact with specialized
letter detectors that are tuned for horizontally aligned sets of
letters (e.g., Dufau, Grainger, & Holcomb, 2008; Grainger,
Granier, Farioli, Van Assche, & van Heuven, 2006). The output
of these detectors may reduce the need to maintain a low-level
visual representation in VSTM, which is, in turn, reﬂected in the
smaller SPCN for words than for nonwords, observed in the
present empirical context. At the neurophysiological level, it is
natural to hypothesize that the present selective attenuation of an
SPCN response to words versus nonwords may arise from the
short-range bidirectional connections between the extrastriate
portion of the visual cortex, the lower part of the parietal lobes
(which are held to be primary sources of SPCN activity) and
regions in the surroundings of the left lingual and fusiform gyri,
these latter regions known to be involved in early visual processing of word-like stimuli (e.g., Nobre, Allison, & McCarthy,
1994; Puce, Allison, Asgari, & McCarthy, 1996).
At ﬁrst blush, the absence of an effect of pronounceability
seems surprising, especially in light of the behavioral literature
suggesting that some form of phonological processing involvement, even when pictures are displayed, may occur rapidly and
automatically (e.g., Morsella & Miozzo, 2002). It is possible we
did not have the statistical power to detect such phonological
effects in terms of an impact of the manipulation of pronounceability on the SPCN amplitude, despite the large sample of subjects tested in each of the present experimental conditions.
However, if this were not lack of power, a different possibility is
that VSTM mechanisms are not subject to the inﬂuence mediated
by phonological recoding, and this may not be surprising given
the long studied sensitivity of this memory subsystem to spatial,
and not verbal, information about objects. Evidence concerning
the neurophysiology of word processing may help to corroborate
this view. Contrary to the topographical proximity and massive
interconnection of cortical regions in the human brain devoted to
early orthographic processing and the hypothesized substrate of
SPCN activity, regions devoted to the assembly of phonological
codes (i.e., primarily, cortical regions in the left inferior frontal
cortex; e.g., Mado Proverbio, Vecchi, & Zani, 2004) seem to entertain short-range connections with regions in the left temporal
lobe and, however, a restricted bandwidth reentrant circuitry to
the occipito-parietal areas under scrutiny in the present context.
One note of caution is in order, however, concerning the
proposed model for the effects found in our study. We are interpreting the lexical-status effects on SPCN as evidence for interactions between lexical status and VSTM. An alternative
interpretation could be formulated on the basis of results suggesting that other ERP components, such as the ‘‘recognition
potential’’ (RP) are also sensitive to the difference between words
198
and nonwords (e.g., Martı́n-Loeches, Hinojosa, Gomez-Jarabo,
& Rubia, 1999; Rudell & Hua, 1997). The RP is usually observed
as an occipito-parietal negativity peaking in a time window of
200–250 ms following the presentation of words at ﬁxation. The
RP to word stimuli is particularly pronounced over left posterior
occipito-parietal regions (e.g., at P7), probably originating from
the left portion of basal extrastriate cortex labeled visual word
form area (Cohen et al., 2000). Could the SPCN results we report
in this article be a reﬂection of the RP component? This possibility appears to be remote in light of the known functional
properties of the RP and the time course of that component.
Similar to the MEG activity reported by Pulvermüller et al.
(2001), the RP tends to be larger as the semantic richness of word
stimuli is increased (e.g., Hinojosa, Martı́n-Loeches, Muñoz,
Casado, & Pozo, 2004; Martı́n-Loeches, Hinojosa, FernándezFrı́as, & Rubia, 2001), with the degree of semantic association
(or semantic redundancy) of speciﬁc word stimuli positively correlated with the negativity in the RP time range. This appears
incompatible with the present results, given that lexical activation
(words relative to nonwords) in the present case produced a decrease in SPCN amplitude, rather than an increase. Furthermore,
the RP component has a time course that is relatively short
compared with the sustained nature of the SPCN, and a topographical distribution (larger over left posterior recording sites)
that is not consistent with the symmetrical distribution of SPCN
activity we have documented. Although it is clear that there are
other ERP components that are sensitive to the lexical status of
letter strings, the present results appear to be a novel manifestation of processing differences between words and nonwords
that is speciﬁcally related to differences in VSTM processing
efﬁciency (or load) for words and nonwords.
In contrast to the results obtained by McCollough et al.
(2007) for simple geometric shapes, the amplitude of the SPCN
was affected by the spatial conﬁguration of the target letters in
the present experiment. The amplitude of the SPCN was substantially larger in the scrambled condition than in either of the
other two conditions and particularly than in the nonword condition, despite the use of letter strings with identical left-to-right
structure (e.g., consonant, consonant, vowel, as in SPI). This
result is open to a number of interpretations. One of the regions
held to contribute to SPCN activity (i.e., the intra-parietal sulcus,
IPS) has been shown to increase in activity when scrambled objects are displayed relative to objects possessing a regular structure (Xu, 2008). In this perspective, the increment in SPCN
activity with scrambled letters could be taken to reﬂect the likely
increase in activation of neurons in IPS for scrambled letters
versus aligned letters. This possibility, considering the verbal
material used in the present study, is in accored with the idea that
special encoding algorithms are used to process letter strings arrayed horizontally (i.e., via activation of letter detectors that are
tuned for horizontally aligned sets of letters). A second interpretation, which relates to the topographical distribution of neurons
in the visual cortex, is that differences in absolute spatial locations across the scrambled and nonscrambled conditions had an
effect on the amplitude of the SPCN. To clarify, it is possible that
letters encoded from upper visual ﬁeld positions (with early projections to the ventral portion of the occipital lobes) may have
produced a smaller SPCN than letters encoded from lower visual
ﬁeld positions (with early projectons to more dorsal portions of
the occipital lobes), and these effects may be nonlinear, leading to
a net increase in SPCN amplitude when positions off the horizontal midline are used. The horizontal letter strings were always
D. Predovan et al.
on the horizontal midline, and thus caution is mandatory under
these circumstances.
An intriguing aspect of the present results is that the SPCN
elicited by scrambled letters began earlier than the SPCN elicited
by horizontally aligned letter strings. Furthermore, there was
also an increase in SPCN amplitude for scrambled letters relative to horizontally aligned letter strings. One possibility is that
these differences were not tied to the SPCN per se, but rather
arise from differences in the amplitude of the attention-sensitive
N2pc component that is typically observed in the latency range
preceding the onset of the SPCN (e.g., Jolicœur, Brisson, &
Robitaille, 2008). On two thirds of the trials, in the word and
nonword conditions, the three target letters were presented at
known and adjacent locations. In contrast, in the less frequent
scrambled condition, the letters were presented at nonadjacent
and unpredictable locations. Consequently, when scrambled, the
letters were less likely to fall in the same receptive ﬁeld of inferoparietal neurons (Jack et al., 2007). These considerations raise
two possibilities. The ﬁrst possibility is that attention needed to
be oriented to each of the target letters in the scrambled condition, in constrast with a single attentional shift for horizontal
letter strings. Such an increase in attentional demands may have
resulted in an increase in N2pc amplitude, which, in turn, would
have given the impression of an earlier onset for the SPCN. The
second possibility is that scrambled letters were processed as
distinct objects more frequently than horizontally aligned letters,
which may have been processed as a group. If the amplitude of
the SPCN varies as a function of the number of encoded groups,
rather than as a function of the number of letters, then we would
expect the amplitude of the SPCN to be higher for the scrambled
condition relative to the other two conditions (as was found).
Perhaps a larger number of distinct objects to be encoded lead to
a stronger activation of the neurons giving rise to the SPCN, and
a more rapid rise in the activity of these neurons (leading to an
earlier SPCN). An additional and interesting perspective is to
consider the SPCN onset latency difference between scrambled
versus aligned letters not as an effect of onset anticipation for
scrambled letters, but rather as a delay in onset latency for wordlike stimuli. This alternative perspective would not be incongruent with the view proposed above that the activation of letter
detectors may be triggered upon the presentation of horizontally
aligned letter strings, on the assumption that this additional stage
may be time-consuming.
Conclusions
From the point of view of encoding and maintenance in VSTM,
all letter strings are not equal. Letter strings forming words elicit
a smaller SPCN than strings forming nonwords, suggesting
strongly that maintaining word representations taxes VSTM
mechanisms to a lesser extent relative to nonword representations. In addition, the spatial layout of letter strings may also
affect ensuing retention loads in VSTM. Both effects are interesting and suggest that the SPCN component may provide a
powerful new tool to study interactions between early sensory
and memory systems (i.e., VSTM) and processes that interpret
stimuli in the context of rich lexical and semantic representations
in long-term memory engaged during reading. We expect that the
present ﬁndings will lead to important discoveries concerning the
representation of words and nonwords in individuals with and
without reading difﬁculties.
199
Lexical status and VSTM
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(Received January 7, 2008; Accepted May 5, 2008)